Construction of microelectromechanical or microelectronic devices often requires moving a delicate thin film structure created on a source substrate to a new position on a target substrate, with the source substrate being permanently separated from contact with the thin film structure. Various lift procedures based on low tack adhesives or electrostatic forces have been developed to allow conveyance of thin film structures between different substrates. For example, thin metallic leads carried on adhesive tape are often used for production of semiconductor devices in various tape bonding processes.
Unfortunately, the stressful process of separating the thin film structure from adhesive attachment to a source substrate can deform, alter, or misposition the thin film structure. While this deformation is inconsequential for relatively large and thick electrical contacts, even slight deformation or mispositioning of moving or non-moving elements in complex microelectromechanical devices can result in device failure. For example, micromechanical fluid valves composed of thin, movable metallic or polymeric structures are particularly susceptible to curling deformations induced during the separation process. When arrays of such valves are produced and assembled, even failure of a handful of valves due to deformation destroys or greatly diminishes utility of the entire valve array assembly.
The present invention alleviates some of the problems associated with the foregoing methods for transferring thin film structures by providing a low stress transfer method based on use of non-adhesive polymeric membranes. In the method of the present invention, a thin film is affixed to a low tack polymeric membrane. While positioned on the polymeric membrane, the thin film is machined to define a thin film structure. This thin film structure (or array of thin film structures) is then separated from the polymeric membrane in a substantially deformation free state. In this manner, various target substrates, including glass, silicon, or printed circuit boards, can be equipped with substantially stress free thin film structures suitable for use in a wide variety of microelectromechanical or microelectronic devices.
The polymeric membrane can be formed from various chemically inert polymeric materials. For best results, low tack elastomeric membranes formed from polysiloxanes, polyurethanes, urethanes, styrenes, olefinics, copolyesters, polyamides, or other melt processible rubbers can be used. For example, a room temperature vulcanizable polysiloxane such as Sylgard 184, manufactured by Dow Corning Corp., can be used. Suitable membranes are also available from Vichem Corporation, Sunnyvale, Calif., under the trade name GEL-PAK.TM..
For sufficiently small pieces, the polymeric membrane can be unsupported. For use in conjunction with larger pieces or large batch fabricated arrays of microstructures, the optional use of a support layer capable of rigidly or flexibly supporting the polymeric membrane is preferred. For example, glass, sapphire, or epoxy impregnated fiberglass laminates such as used in conventional printed circuit boards can be formed as a suitable rigid support layer, while various polymeric materials such as polyesters, polyamides, polyimides, polyolefins, polyketones, polycarbonates, polyetherimides, fluoropolymers, polystyrene, or polyvinyl chloride can also optionally be used as either a rigid or flexible membrane support layer.
Advantageously, both the polymeric membrane and any optional membrane support layer can be selected to be transparent or substantially transparent. Transparency allows a user to optically guide movement of thin film structure supported on a source substrate into an appropriate position with respect to a target substrate. In addition, optical transparency simplifies use of laser cutting or etching techniques and allows for quality control inspections of both sides of the thin film structures prior to mating with the target substrate. In one particularly useful embodiment, thin film structures defined on a metallized polymeric film sandwiched between two polymeric membranes can be machined by lasers that transfer energy to cut the metallized polymeric film without transferring energy to cut the sandwiching transparent polymeric membranes.
As will be appreciated, in addition to lasers, various mechanical, electrical, chemical, acoustic, or optical techniques can be used to machine, define or modify structures in the thin film layer. For example, mechanical techniques can include stamping, die cutting, kiss cutting, shearing, punching, blanking, forming, bending, forging, coining, upsetting, flanging, squeezing, and hammering using presses with a movable ram that can be pressed against the supported thin film layer. Electrical techniques can include electrical discharge machining using high frequency electric sparks. Chemical techniques are commonly employed in conjunction with electrical or mechanical techniques, and can include chemical/mechanical polishing, electrochemical machining using controlled dissolution of metals, electrolytic grinding, electrochemical arc machining using controlled arcs in an aqueous material to remove thin film material, and acid electrolyte capillary drilling. Acoustic techniques such as ultrasonic machining using abrasives, or ultrasonic twist drilling are also suitable for shaping the thin film, as are optical techniques such laser cutting and drilling or various patterning techniques using photochemical resist etching. In certain embodiments, high pressure fluid drilling or cutting (with or without entrained abrasives) can even be used.
As will be appreciated, the present invention has particular utility in conjunction with applications requiring the use of sensitive and fragile thin films (organic, inorganic, or composite films). Unlike their bulk counterparts, thin films are extremely sensitive to applied stress. Particular areas of concern in the handling and processing of thin films include wrinkling, creasing, scratches, contamination, and residual/surface stresses. The first issue is handling damage. For example, when dealing with metallized polymers (like the 0.005" aluminized polyesters), rolls of the material are preferred to reduce any manual handling of the thin film. Any creasing, wrinkling or folding of the thin film may permanently damage the film. However, before proceeding onto the actual fabrication steps utilizing a thin film, it must generally be mounted or otherwise held down. Lamination of the thin film onto sheets of conventional transfer mats (e.g. an adhesive acrylic/paper composite used in die cut and stamping operations) results in stress and deformation of the film upon removal. The accumulation of residual stress results in catastrophic deformation to the thin film samples. Upon release from the transfer mat, the thin film structures curl up upon itself in an attempt to minimize surface energy.
However, use of the method of the present invention advantageously results in minimal induced stresses upon release of the completed thin film structures, allowing safe transfer and handling of even large thin film sheets, thin film structures, or arrays of thin film structures. Use of a polymeric or elastomeric material in accordance with the present invention also simplifies machining and fabricating processes when used as a sacrificial layer, and can facilitate optical alignment methods during transfer and attachment processes. Additionally, the use of electrically conductive adhesives permit thermoset and thermoplastic heat reactions for bonding thin film structures to the target substrate, widening material choices that accommodate thermal budgets of a device, and even providing the ability to reposition or rework an attached thin film structure by reheating the thermoplastic adhesive on the target substrate.
Given the foregoing advantages, those skilled in the art will appreciate that while useful for production of electrical contacts, pads, leads, transistor elements, dielectric caps, or other conventional microelectronic elements, thin film structures produced in accordance with the present invention are particularly valuable for use in microelectromechanical systems (MEMS). Example MEMS devices can include microoptical systems such as lenses, waveguides, diffraction gratings, semiconductor laser arrays, or light detectors. Other MEMS devices can include microactuators, mechanical filter systems, acoustic or vibration sensors based on cantilevered structures, thermal sensors, or even arrays of electrically actuated valves, such as represented by electrostatic or electromagnetic flap valves used for fluid control. The present invention has particular utility for production of movable elements (or support structures for movable elements) sized on the order of microns to millimeters.
Additional functions, objects, advantages, and features of the present invention will become apparent from consideration of the following description and figures of preferred embodiments.